1. Field of the Invention
The present invention relates to concrete and a method of making the same and, more particularly, to a concrete and its method of manufacture wherein relatively larger quantities of fly ash can be incorporated in the concrete than heretofore thought possible.
2. The Prior Art
The prior art teaches the making of concrete from Portland Cement, aggregate (generally a mixture of a fine aggregate such as sand and a coarse aggregate such as gravel or crushed rock), and water. A typical concrete, for example, could be made from 470 pounds of Portland Cement, 1612 pounds of sand, 1736 pounds of crushed rock and 250 pounds of water. The relative portions of Portland Cement, sand and rock will vary depending upon the requirements for the end product. The prior art teaches the employment of organic and inorganic additives to improve or vary the properties of the resulting concrete end product.
It has also been proposed in the past to substitute fly ash for certain portions of the Portland Cement. The term "fly ash" as used herein, is intended to indicate the finely divided ash residue produced by the combustion of pulverized coal or lignite, which ash is carried off with the gases exhausted from the furnace in which the coal or lignite is burned and which is collected from these gases usually by means of suitable precipitation apparatus such as electrical precipitators. Those finely pulverized ashes resulting from combustion of oil or from combustion of waste materials in a large incinerator, or natural pozzolans, can also be utilized as "fly ash" providing their chemical compositions are reasonably similar to fly ash produced by the combustion of pulverized coal or lignite. Fly ash is generally obtained in a finely divided state such that usually, at least 70% by weight passes through a 200-mesh sieve, although incinerator ashes may be considerably coarser. Fly ash may be considered an "artificial pozzolan," as distinguished from a "natural pozzolan."
Fly ash, for the most part, is considered to be a waste product, whereas, Portland Cement is a relatively expensive product and, sometimes, in short supply. Thus, the substitution of fly ash for a portion of the Portland Cement in a concrete mix will serve several desirable ends: first of all, it will reduce the cost of making the concrete; secondly, it will tend to conserve the supply of Portland Cement; and, thirdly, it will utilize a waste product.
On a straight substitution basis, generally no more than about 20% of the Portland Cement can be replaced by fly ash; otherwise, the resulting concrete product is inferior or unsuitable as a structural concrete.
There is much in the literature that deals with the properties of Portland Cement and the chemical reactions which occur during the formation of concrete. Briefly stated, however, Portland Cement is generally considered to be comprised of lime, silica, alumina and iron oxide. In the United States, Portland Cement, as purchased, generally contains about 2% by weight of gypsum (calcium sulphate) which is added as a retarder. When Portland Cement is mixed with water, a chemical reaction begins between the various compounds and the water. In the initial stages, the small quantity of retarder quickly goes into solution, and is, thus, enabled to exert its influence on the other chemcial reactions which are starting. These chemical reactions result in the formation of the various compounds which cause setting and hardening, the four most important being: tricalcium aluminate which hydrates very rapidly and produces a considerable amount of heat, causing initial stiffening but contributing the least to the ultimate strength; tricalcium silicate, which jellifies within a few hours, generating considerable heat and having a marked effect on the strength of the concrete in its early stages, mainly in the first fourteen days; dicalcium silicate whose formation proceeds slowly with a slow rate of heat evolution and which is mainly responsible for the progressive increase in strength which occurs from fourteen to twenty-eight days and beyond; and, tetracalcium alumino-ferrite which appears to have no marked effect on the strength or other properties of the hardened cement.
Fly ash is similar to Portland Cement in that it also contains certain amounts of lime, silica, alumina and iron oxide. However, where Portland Cement normally contains preferably, about 60% by weight of lime, fly ash fly will contain from 5 to 30% of lime depending upon the source of the fly ash. Again, where Portland Cement contains, preferably, about 20% silica, fly ash will contain from 20 to 60% by weight of silica, depending upon its source. As far as alumina is concerned, Portland Cement will normally include less than 10%, say about 6%, whereas fly ash will contain from 15 to 30% alumina. Finally, with regard to iron oxide, both the Portland Cement and the fly ash contain about less than 10%, with the Portland Cement averaging between 3 and 4% iron oxide and the fly ash averaging about 7 to 8% iron oxide. The fly ash, in addition, contains further impurities in the nature of sulphur trioxide and magnesium oxide.
On the surface, at least, it would appear that fly ash had similar cementitous properties and could, therefore, be substituted as desired for Portland Cement. (This is disregarding the physical properties of the fly ash as compared to the physical properties of Portland Cement.) However, in practice, fly ash is not fully substitutable for Portland Cement. In the ready-mix concrete industry, it is recommended that no more than 20% of the Portland Cement be replaced by fly ash. The literature suggests that the maximum substitution of fly ash should not exceed 50%. The reason why higher percentages of fly ash cannot be used in concrete is perhaps, not fully understood. It has been theorized that one of the phenomena which limits the quantity of fly ash in cement is the occurance of inert coats on the fly ash particles. The coating on the fly ash particle is believed to occur during the process of burning the coal or lignite so that it does not allow the considerable pozzolanic energy contained in the fly ash to be utilized.
The above comments regarding the possible substitution of fly ash for Portland Cement in concrete have basic reference to untreated fly ash. However, the prior art suggests that the amount of fly ash which can be incorporated into concrete can be relatively increased where the fly ash is treated by various chemical, thermal or mechanical techniques or combinations of these techniques.
The prior art also teaches that the properties of concrete can be improved or modified by adding an organic material to the concrete mix. One such organic additive is a styrene-one, three butadiene copolymer latex emulsion; other organic additives include a polyvinylidene chloride latex, methyl methacrylate, styrene, a copolymer of methyl methacrylate and styrene, and other similar organic materials.